Electron beam devices, in particular scanning electron microscopes, are used for examining surfaces of objects (samples). To do so, an electron beam (hereinafter also referred to as a primary electron beam) is generated by an electron gun in a scanning electron microscope and is focused by the objective lens on the object to be examined. Using a deflecting device, the primary electron beam is guided in a grid pattern over the surface of the object to be examined. The electrons of the primary electron beam interact with the object. As a result of this interaction, electrons in particular are emitted from the object (so-called secondary electrons) or electrons of the primary electron beam are backscattered (so-called backscatter electrons). The backscatter electrons have an energy in the range of 50 eV up to the energy of the electrons of the primary electron beam on the object, while the secondary electrons have an energy of less than 50 eV. Secondary electrons and backscatter electrons form the secondary beam, as it is referred to below, and are detected by a detector. The detector signal generated in this way is used for image production.
Electron beam devices have a high positional resolution which is achieved due to the very small diameter of the electron beam in the plane of the object. The resolution is better, the closer the object is situated to the objective lens of the electron beam device. For detecting the secondary electrons or backscatter electrons, the detector is preferably situated inside the objective lens or in an area between the objective lens and the electron gun. Furthermore, the resolution can be improved, in particular with a primary electron energy of less than 10 keV by first accelerating the electrons of the primary electron beam in the electron beam device and then decelerating them to a desired final energy in the objective lens or in the area between the objective lens and the object. Such an electron beam device is described in DE 198 28 476 A1, for example.
There are known electron beam devices having an annular detector situated around the beam axis of the primary electron beam and having an opening, which is normally much larger than the beam cross section of the primary electron beam, so as not to influence the primary electron beam in the beam path of the electron beam device. The return paths of the secondary electrons and backscatter electrons in the electron beam device are influenced in different ways by the objective lens due to the different energies of the secondary electrons and backscatter electrons. The crossover of the beam of the secondary electrons occurs closer to the object to be examined than the crossover of the beam of the backscatter electrons. Under certain operating conditions, in particular at a high magnification and with a small working distance between the objective lens and the sample, the secondary and/or backscatter electrons travel on paths in such a way that a majority of the secondary electrons and backscatter electrons pass through the opening in the detector and therefore are not detected.
DE 198 28 476 A1 describes one possible approach for overcoming the disadvantage described above. With the electron beam device known from this publication, two detectors for the secondary electrons and the backscatter electrons, each having one opening, are offset from one another in the direction of the optical axis of the electron beam device. The first detector, which is situated near the object, is for detecting the electrons emerging from the object at a relatively large angle while the second detector, which is situated in the area of the electron gun, is for detecting the electrons that emerge from the object at a relatively small angle and pass through the opening in the first detector provided for passage of the primary electron beam.
Another proposed approach is known from EP 0 661 727 A2. This document relates to a scanning electron microscope in which secondary electrons and backscatter electrons are deflected on various paths to different detectors via a Wien filter. As an alternative, it is proposed that the detector be designed as a conversion electrode with which it is possible to detect either backscatter electrons or secondary electrons individually or simultaneously via a single detector.
Another Wien filter system is known from WO 00/36630. This document relates to a scanning electron microscope having two or three successive Wien filters that are used to deflect the Auger electrons emitted from an object. Quadrupole fields are superimposed on the Wien filters to compensate for the imaging errors caused by the dispersion in the Wien filters.
EP 0 989 584 A1 relates to two Wien filters that are provided for reducing the energy width of the primary electron beam and are designed as quadrupole filters.
Furthermore, EP-A-910 109 A1 describes an objective lens for influencing an electron beam with a magnetic single-pole lens and an electrostatic lens having a first and a second electrode which are equipped with different potentials. The electrostatic lens is situated downstream from the magnetic single pole lens in the direction of the electron beam, one of the two electrodes of the electrostatic lens being designed as a multipole. Moreover, an arrangement of magnetic multipole elements is also provided to form Wien filters together with the electrodes. These may be adjusted so that the primary electron beam is not influenced but any secondary electrons and backscatter electrons released on an object (sample) are deflected and sent to an extra-axial detector capable of detecting both signals jointly or separately due to a suitable design.
The disadvantage of all the Wien filter approaches is the relatively complex implementation in scanning electron microscopes because the beam guidance tube of the scanning electron microscope must usually be segmented.
Accordingly, it would be desirable to simplify and improve detection of electrons backscattered by an object or electrons emitted by an object in the case of an electron beam device.